On Wednesday, California's Department of Motor Vehicles (DMV) told Uber ride-hailing company to stop its self-driving car service for its passengers in San Francisco because it was illegal.

The order by the California DMV came close on the heels of Uber's earlier in the day announcement of the launch of its self-driving car service in San Francisco, making it the second US city - after Pittsburg - where Uber's self- driving car service could be availed by passengers.

In a letter sent to Uber late Wednesday, California DMV Deputy Director Brian G. Soublet said that the company cannot legally offer self-driving car service to San Francisco passengers because it does not have the required state permits for autonomous driving.

Soublet said in the letter that Uber cannot operate its self-driving vehicles on public roads in California "until it receives an autonomous vehicle testing permit;" and added: "Any action by Uber to continue the operation of vehicles equipped with autonomous technology on public streets in California must cease until Uber complies."

The move by California regulators, to ask Uber to stop its self-driving car service in San Francisco, marks a somewhat embarrassing twist for the company which had set up the rollout of its self-driving cars in the city as a big event, and is now facing regulatory hassles.